The present invention relates to propylene polymer compositions which are suitable for moulding, especially injection moulding. More particularly the propylene polymer compositions can be used for packaging applications, especially for deep freeze packaging applications because they have excellent impact strength/stiffness balance at low temperatures, sufficient flowability and good optical properties.
Transparent polypropylene compositions with superior toughness at low temperatures have become an important market trend in recent years. One type of targeted products is rather stiff with flexural moduli well above 800 MPa. Materials with good stiffness are required because structural integrity is necessary for the intended products in the moulding segment, e.g. for boxes, crates, thin wall packaging, etc.
Types of propylene polymers having the required transparency are for example propylene random copolymers with ethylene as comonomer. However, their impact resistance below 0° C. is insufficient and stiffness is below the required level.
There are also propylene polymers known, which have the required impact resistance at low temperatures. These polymers are heterophasic propylene copolymers, having a rubber phase (usually about 20-30 wt % rubber phase with 40-60 wt % propylene) dispersed in a continuous matrix phase. For a good low temperature impact resistance, the molecular weight of the rubber phase is higher than the molecular weight of the matrix phase. These heterophasic propylene copolymers are however very opaque. If the molecular weight of the polymer in the dispersed phase is lower than that of the matrix phase, transparency is increased and impact resistance is decreased. A lower total MFR of the composition results in an increased impact strength, however, the processability of the polymer composition which is required for the intended application (injection moulding) decreases.
It is therefore the object of the invention, to provide a polypropylene composition for moulding, which has low haze and which simultaneously shows a good impact strength/stiffness balance with good impact strengths at low temperatures.